The present invention relates to a method for the preparation of methyl chloride from carbon tetrachloride and methyl alcohol as the starting materials. More particularly, the invention relates to an efficient method for the preparation of methyl chloride from carbon tetrachloride and methyl alcohol by the gas-phase reaction in the presence of a solid catalyst.
Methyl chloride, which is a useful chlorinated hydrocarbon compound consumed in large quantities, for example, in the synthesis of methyl chlorosilanes as the starting materials of silicone products, is produced, as a typical industrial process, by the chlorination reaction of methane which yields not only methyl chloride but also other higher chlorinated methanes including methylene chloride, chloroform and carbon tetrachloride by the successive unit reactions expressed by the reaction equations: EQU CH.sub.4 +Cl.sub.2 .fwdarw.CH.sub.3 Cl+HCl; EQU CH.sub.3 Cl+Cl.sub.2 .fwdarw.CH.sub.2 Cl.sub.2 +HCl; EQU CH.sub.2 Cl.sub.2 +Cl.sub.2 .fwdarw.CHCl.sub.3 +HCl; and EQU CHCl.sub.3 +Cl.sub.2 .fwdarw.CCl.sub.4 +HCl.
These four reactions concurrently proceed in the reaction mixture so that the reaction product obtained by the process is always a mixture of these four kinds of chlorinated methane compounds, from which each of the compounds is to be isolated, for example, by distillation. A problem in this process is that, while the first three chlorinated compounds, i.e. methyl chloride, methylene chloride and chloroform, are useful and consumed in relatively large quantities as compared with carbon tetrachloride, the proportion of these four kinds of chlorinated compounds in the reaction mixture cannot be freely controlled so that overproduction of carbon tetrachloride is always unavoidable in the above described process of chlorination in order to satisfy the demand for the other three chlorinated methanes by continuedly practicing the process.
Therefore, it is an important technological issue to develop a method for converting carbon tetrachloride into some other more useful chemicals since otherwise carbon tetrachloride employed as such would ultimately be released to the atmosphere in the vapor form while, as is well known as a serious environmental problem, carbon tetrachloride is strongly suspected to be responsible for the destruction of the ozone layer in the stratosphere leading to an international agreement to entirely ban carbon tetrachloride by the end of this century reached in the London conference held in June, 1990.
Several proposals and attempts have been made heretofore for the conversion of carbon tetrachloride into other more useful and harmless chemical compounds, of which some are still in the laboratory stage and the others are already industrialized in a certain scale. Firstly, for example, carbon tetrachloride can be reduced with hydrogen in the presence of a catalyst such as Raney nickel into lower chlorinated methanes such as methyl chloride and methylene chloride. This method, however, is disadvantageous as an industrial process due to the low reaction velocity and limited life of the catalyst so that this method cannot be rendered to practice unless these problems could be solved in addition to the problem that by-products such as 1,2-dichloroethane and the like are produced in a large quantity.
Secondly, carbon tetrachloride can be burnt together with a fuel gas such as methane, liquefied petroleum gas and the like and converted into carbon dioxide and hydrogen chloride which can be recovered and utilized. This method is industrially disadvantageous due to the high reaction temperature of 1000.degree. C. or even higher, at which the process of combustion can be proceed, and the corrosiveness of hydrogen chloride requiring a special structure of the furnace and an expensive high-grade corrosion-resistant refractory material for the furnace.
Thirdly, U.S. Pat. No. 4,423,024 proposes a method in which carbon tetrachloride is hydrolyzed in the vapor phase in the presence of a zeolite or molecular sieve catalyst. This method, which is conducted at a temperature of 240.degree. to 330.degree. C., also has a problem in the material of the reactor in addition to the relatively short life of the catalyst due to the deposition of tarry matters thereon as produced by the themal decomposition of carbon tetrachloride.
Fourthly, the inventors have previously proposed a method for the direct conversion of carbon tetrachloreide into methyl chloride by a single step reaction of carbon tetrachloride with methyl alcohol in the presence of a solid catalyst such as zinc chloride supported on an active carbon carrier. A problem in this method is the difficulty in the control of the reaction temperature because the reaction is highly exothermic generating about 70 kilocalories of the heat of reaction per mole of the starting carbon tetrachloride resulting in local heating of the catalyst bed into which carbon tetrachloride and methyl alcohol are introduced jointly to disturb smooth proceeding of the reaction.
Thus, none of the above described methods is industrially quite satisfactory in one or more respects for the conversion of carbon tetrachloride into more useful and harmless chemical compounds such as methyl chloride.